This invention relates to a coordinate reader for inputting coordinate values to an external apparatus such as a computer, and more specifically to a wireless coordinate reader in which a coordinate reader main body need not be connected to a coordinate indicator by signal lines.
In a coordinate reader which influences electromagnetic coupling between an excitation line and a sense line through a resonant circuit consisting of a coil and a capacitor and provided as part of a coordinate indicator, the phase of the induction signal induced to the sense line changes with the resonance frequency of the coordinate indicator.
Referring to FIG. 14(b), the resonance frequency of the coordinate indicator can be changed, for example, by connecting a series circuit formed by a switch 107 and a second capacitor 108 in parallel with a basic parallel resonant circuit formed by a coil 105 and a first capacitor 106.
As shown in FIG. 14(a), the coordinate indicator 104, constituted as described above, is associated with a tablet, or plate, 101 which includes excitation lines 102 and sense lines 103 disposed so as to cross one another orthogonally. An A.C. signal is applied as an excitation signal to the excitation lines 102 and the induction signals induced on the sense lines 103 are observed.
The coordinate indicator 104 is placed on or directly above the sense line plate 101 and the switch 107 is turned ON or OFF. When the switch 107 is OFF, the induction signal and the excitation signal have a predetermined phase difference due to the effect resulting from electromagnetic coupling and to the circuit characteristics. When the switch 107 is ON, the second capacitor 108 is connected in parallel with the resonant circuit, so that the resonance frequency changes to a lower frequency value and the phase of the induction signal has a greater delay than at the time switch 107 is OFF. The state of switch 107 can be judged by detecting this phase delay.
In the general structures of coordinate readers, a circuit substrate or the like is disposed often below the sense line plate and a shield plate consisting of a metal plate is disposed in most cases between the sense line plate and the circuit substrate in order to cut off noise.
If the metal plate is disposed below the sense line plate, the inductance of the coil of the coordinate indicator is affected by it. The inductance of the coil changes in accordance with the height at which the coordinate indicator is placed or, in other words, in accordance with the distance between the coordinate indicator and the shield plate. This change of coil inductance affects the phase of the induction signal.
This becomes a problem for a system which judges the state of the switches of the coordinate indicator by the change of phase of the induction signal. Since the phase difference between switches becomes inevitably smaller when a greater number of switches are to be detected, the change of phase due to indicator height cannot be neglected.